1. Field of the Invention
The present invention relates to a drum brake device for use in braking vehicles. More particularly, the invention relates to a drum brake device which functions such that during a service brake application, it operates as a brake device of the leading/trailing type having a good ability of producing stable braking effect, and during a parking brake application, it operates as a brake device of the duo servo type having a good ability of producing high braking effect.
2. Description of the Related Art
Generally, the drum brake device may be categorized into brake devices of the leading/trailing type, the two-leading type and duo servo type.
In the drum brake of the leading/trailing type, a wheel cylinder for expanding brake shoes is disposed between a pair of opposed brake shoes, specifically at one ends of those opposed brake shoes. An anchor portion is disposed between the other ends of the brake shoes. One brake shoe is used as a leading shoe, while the other brake shoe is used as a trailing shoe.
In the drum brake of the two-leading brake type, a wheel cylinder is disposed between the opposed ends of a pair of brake shoes oppositely disposed. The brake shoes are both used as leading shoes. This type of drum brake device is further categorized into a brake device of the single-drive type (TP1W), and a brake device of the dual-drive type (TP2W).
In the drum brake device of the duo servo type, a wheel cylinder for expanding brake shoes is disposed between a pair of opposed brake shoes, specifically at one ends of those opposed brake shoes. The other ends of the opposed brake shoes are mutually coupled by a link mechanism for transmitting an anchor counter force from one brake shoe to the other brake shoe.
Of those types of drum brake devices, the leading/trailing type drum brake device is advantageous in that the braking effect is stable, and a parking brake may readily be assembled into the brake device, but is disadvantageous in that the braking effect is lower in magnitude than that of the drum brake devices of the two-leading and duo servo type. Of the two-leading type of the drum brake device, the single-drive type (TP1W) brake device produces high gain during forward braking. During reverse braking, the drum brake device functions as a two-leading type brake device, and the braking effect is likely to be lower than the braking effect produced when it operates during forward braking. The dual-drive type (TP2W) brake device produces high braking effect during both the forward and reverse braking, and hence produces high gain. However, two wheel cylinders are indispensably used. Use of the two wheel cylinders increases manufacturing cost of the brake device, and requires a complicated mechanism for the parking brake assembled thereinto.
In the drum brake device of the duo servo type, an anchor counter force is input from one brake shoe to the other brake shoe. In other words, this drum brake device has the servo function. The natural results are that it can produce considerably high braking effect in both the forward and reverse braking modes, size reduction of the brake device is easy, and assembling the parking brake into the brake device is easy.
A serious disadvantage of the brake device of the duo servo type is that the braking effect greatly varies with friction coefficient of the brake shoe lining and contact states of the rotary drum. viz., the braking characteristics are instable. This is due to the fact that the brake device is sensitive to variations of the lining friction coefficient and the contact states.
Thus, those types of drum brake devices have advantages and disadvantages. The usual choice usually given to the designer in this circumstance is to appropriately select the type of drum brake device from among those ones in accordance with the performances and use purposes of the vehicles to which the brake device is applied and production scales of the vehicles.
For the service brake application, the brake device is operated, and the wheel cylinder responsively operates and expands the brake shoes to thereby generate a braking force. During the service brake application, fine control of the braking force is required. Therefore, stability of the braking effect is essential. During the parking brake application, a mechanism interlocked with a parking lever, not the wheel cylinder, forcibly expands the brake shoes to generate the braking force. Therefore, reliable braking of the vehicle is essential, and hence, production of high braking effect is required. Thus, the braking characteristic required for exercising the service brake application is contradictory to that for exercising the parking brake application.
However, it is very difficult to realize coexistence of the contradictory braking characteristics in satisfactory levels in one drum brake device of the leading/trailing, two-leading or duo servo type.
To overcome the technical difficulty, a dual mode drum brake device is disclosed in JP-A-3-113133. In the brake device, during the service brake application, a pair of brakes shoes are operated in a leading/trailing mode to produce a required stability of the braking effect. During the parking brake application, those brake shoes are operated in the duo servo mode to produce high braking effect.
The parking brake mechanism, which is used for the dual mode drum brake device of JP-A-3-113133, uses a large lever member that ranges between the paired brake shoes. Therefore, large component parts must be used for constructing the parking brake mechanism.
Further, in assembling, the component parts must be mounted on both the brake shoes, and the assembling work of the parking brake mechanism is inefficient.